Field of the Invention
The invention relates to a current-mode switching regulator, in particular for pulsed power supplies and/or switched-mode power supplies.
Switching regulators can be used in a multiplicity of different application possibilities, such as e.g. in pulsed power supplies and switched-mode power supplies. In the case of a pulsed power supply, an input voltage—for example a rectified mains voltage—is converted into an essentially rectified, pulsed DC voltage with a variable duty ratio by a pulse width modulator and a power switch connected downstream thereof. The task of the switching regulator is to keep the voltage at the output of a filter of the power supply constant over the widest possible output current range. Furthermore, the switching regulator must be configured to detect the output current and the output voltage of the filter and to generate therefrom a control signal for driving the power switch.
Essentially three different control strategies exist for the control of such switching regulators: voltage mode control, feedforward control and current mode control. The control strategies are described for example in the reference by R. E. Tarter, titled “Solid-State Power Conversion Handbook”, Wiley Interscience, New York, 1993.
Current mode control constitutes a particularly elegant and effective type of control which, moreover, requires only comparatively small compensation capacities and is thus ideal for use in integrated switching regulators.
Current-mode switching regulators have an inner control loop for controlling the load current and an outer control loop which, together with the inner control loop, serves for controlling the output voltage. With the switch switched on, the current rising in ramped fashion through an inductor coil of the switching regulator causes the voltage across a measuring resistor likewise to have a ramped shape. When a desired voltage prescribed by the outer control loop is reached, the switch switches off.
In most voltage regulators, for example in linear regulators or switching regulators, it is often desired to increase the output voltage of the regulator e.g. by devices or components that can be provided externally, in order thereby to ensure a more flexible area of use for the voltage regulator. In the case of voltage regulators configured as linear regulators, this can be achieved very simply by connecting a voltage divider upstream of the input with which the linear regulator monitors the amplitude of the output voltage. Such a control strategy is also referred to as a feedback divider.
Such a measure is also possible, in principle, in switching regulators. However, in switching regulators which are operated according to so-called current-mode control, care must be taken to ensure that the additional ramp added to the current ramp in order to avoid subharmonic oscillations, for the so-called slope compensation, is adapted to an altered output voltage. In this case, however, it must be taken into account that, on the one hand, the amplitude of the load current ramp increases when the output voltage increases, but it must always remain significantly smaller than the supply voltage of the circuit blocks involved, since otherwise the advantages of current-mode control disappear. On the other hand, the amplitude of the current ramp must also not become too small, in order to ensure interference immunity with respect to interference signals.